The present invention relates to power amplifiers, and more particularly to power amplifier systems in which multiple signals are simultaneously transmitted at varying power levels.
In electronic communication systems, it is often necessary that groups of information signals be amplified and transmitted simultaneously. For example, a cellular radio base station transmitter typically transmits signals to many active receiving mobile stations within a single geographic cell. The signals typically appear at multiple predetermined frequencies in such multi-carrier signals. Similarly, a satellite communications transponder amplifies and transmits large number of information signals destined for various participating remote stations. Because such systems customarily employ a frequency division multiple access (FDMA) scheme, in which information signals are modulated on signal carriers occupying several frequency channels within an allocated frequency band, care must be taken to avoid inter-channel interference which may corrupt signal transmissions.
One possible source of such cross-channel interference is known as intermodulation distortion (IMD), which may result when two or more signals of different frequencies are mixed. For example, if two carriers of different frequencies are amplified using a non-linear amplifier, spurious outputs occur at the sum and difference of integer multiples of the original carrier frequencies.
Third order intermodulation products resulting from two relatively strong signals may disrupt transmission of a third relatively weak signal being transmitted on a carrier having a frequency equal to the frequency of the intermodulation product. It is desirable to reduce this distortion.
Base station power amplifier systems exhibiting a high degree of linearity can be desirable since such amplifiers can help minimize the out-of-band emissions and allow for frequency reuse schemes. Amplifier systems are made up of power amplifiers (or “gain blocks”) to provide more gain than possible with a single gain block.
Various solutions have been proposed for improving linearity and reducing inter-channel effects in multi-carrier amplifier systems. One such solution is the feed-forward amplifier system. To further improve linearity in a feed forward amplifier system predistorters may be added in front of individual power amplifiers (gain blocks), or the bias applied to gain blocks may be controlled.
In the feed-forward amplifier system two loops can be used to cancel distortion. In a first loop, a portion of the signals at the input to the amplifier are fed forward and, following suitable amplitude and phase adjustment, are subtracted from the amplifier output to generate an error signal. The error signal is proportional to distortion components of the output. The first loop that generates the error signal is known as the signal-cancellation loop. The error signal is then amplified, phase-adjusted and subtracted from the amplifier output to give a corrected signal output with reduced distortion effects. This portion of the circuit is known as the error-cancellation loop.
Nonlinearity can be tolerated in a feed forward amplifier system if the input signal to the amplifier has a constant envelope. However, problems can arise when the input signal has a large peak-to-peak average or signal envelope.
In some recent digital modulation schemes with improved bandwidth efficiency, information is embedded only in a carrier phase. Nevertheless, the input signal is typically characterized by a relatively large amplitude variation or “signal envelope”, and on average the signal amplitude remains considerably lower than the frequent signal peaks. To help control amplitude distortion and out of band emissions, it is desirable for power amplifiers to be capable of handling these frequent signal peaks and remain somewhat efficient in their power consumption. A way to boost an amplifier's ability to faithfully reproduce large dynamic range signals and maintain a degree of efficiency is to switch in a greater power supply voltage when it is needed. The power amplifier is required to switch between these peaks. However, in the prior art switch losses tend to negate the benefits of switching.
Conventional approaches have utilized continuous tracking of the signal envelope., However, such approaches can be inefficient and result in significant power losses, particularly when the input signal is characterized by a large average to peak ratio.
Accordingly, there is a need for controlling the bias supply to help reduce intermodulation distortion in a feed forward amplifier system that amplifies input signals that vary considerably in amplitude. It is also desirable to efficiently utilize the power applied to the amplifier such that excessive power is not consumed, and that the amplifier provides a linearity similar to that of an amplifier biased for linear operation. For example, an amplifier that is heavily biased class A or AB.